All crude oil contains impurities which contribute to corrosion, heat exchange fouling, furnace coking catalyst deactivation and product degradation.
Corrosion has always and is currently a significant problem in the refining industry because of the replacement costs and downtime associated therewith. As the industry has expanded and became more complex so have the corrosion problems.
Corrosion problems in a refinery operation and in particular a crude unit can be due to any one of or a combination of (i) those components found in crude oil (ii) the chemicals used in the refinery process, and (iii) environmental conditions. The present invention is directed to those corrosion problems which are due to one of the constituents normally contained in the crude oil or in some cases formed during the actual process. The four main impurities in crude oils which contribute to corrosion of condenser piping, distillation units and other structures of the refinery equipment include salts, sulfur compounds, naphthenic and other organic acids such as acetic and propionic acid, organic and inorganic acids.
The salts hydrolize during processing of the crude to produce hydrochloric acid which is very corrosive. Sulfur compounds are troublesome because they break down into hydrogen sulfide which in fact has the capacity to make the corrosion due to hydrochloric acid even more severe.
Naphthenic acid is a generic term used to identify a mixture of organic acids present in the petroleum stock, or which may be obtained due to the decomposition of the naphthenic and/or other organic acids. Acids of this nature include, but are not limited to, carbonic acetic and propionic acids and mixtures thereof which together with the naphthenic acid cause corrosion at temperature ranges of 150.degree. to 750.degree. F.
Accordingly, corrosion occurs due to those acids in centrifugal pumps, furnace tube inlets and return bends, transfer lines, crude tower flash zones, tower overheads, etc. These acids do not require an aqueous phase to cause corrosion and while in many cases they are not particularly corrosive at lower temperatures, they become much more aggressive at the elevated temperatures. Under these conditions rates as high as 0.35 inch per year have been reported for carbon steel and with aluminum it is also quite high, particularly when anhydrous (dry) conditions are prevelant. Acid neutralization number (mg. KOH/gm) is a quantitative indication of the naphthenic acids present in the crude, thus providing some evidence of the corrosive potential of the crude being processed. Crudes with naphthenic acid concentration of greater than 0.5 KOH/gm of crude appear to either possess or generate high acetic and/or propionic acids, thereby resulting in corrosion in the crude unit overheads.
In view of the foregoing then, the industry is constantly looking for techniques and inhibitors to combat corrosion caused by the various constituents contained in the oil being processed and by the chemicals formed during the processing and refining of petroleum and hydrocarbons.